In recent years it has become increasingly apparent that some amino acids previously classified as “dispensable” or “non-essential” can have significant impacts on fish when supplemented in diets. Included in this group are glutamine, glycine, hydroxyproline and proline. Other amino compounds or derivatives of amino acids such as taurine and creatine, which are abundant in fishmeal, also have gained increased attention as efforts continue to identify feedstuffs to potentially replace some portion of fishmeal in the diets of aquatic species to increase the sustainability and cost-effectiveness of aquaculture. However, many alternative protein feedstuffs are deficient in one or more indispensable as well as dispensable amino acids. Therefore, this laboratory intensified its efforts in recent years to evaluate dietary requirements for various amino compounds with red drum (Sciaenops ocellatus), hybrid striped bass (Morone chrysops x M. saxatilis) and channel catfish (Ictalurus punctatus).
Results from several different studies will be presented to provide detailed information about the synthesis of the amino acids traditionally classified as dispensable and how dietary supplementation may improve various metabolic responses. In particular, the presentation will review studies in which dietary glutamine supplementation was evaluated in terms of gastrointestinal health and immunological responses of the species noted above. Supplementation of glutamine at 1 to 2% of diet had beneficial effects on intestinal health and various immunological responses including enhanced vaccination efficiency. Additional studies were conducted with hydroxyproline nutrition of red drum because that amino acid is a vital component of collagen, and growth of other fish species has benefited with its supplementation. However, in our studies with red drum, we were unable to confirm beneficial effects of crystalline hydroxyproline supplementation in regards to growth of fish fed diets high in plant-protein feedstuffs. Changes in hydroxyproline content were found in plasma and tissues at higher inclusion levels, which could have some effect on the texture of fish flesh. Several investigations in this laboratory have established the dietary essentiality of taurine to red drum as well as its potential sparing effect on methionine in other fish species. The positive effects of dietary creatine supplementation on weight gain, feed efficiency, and reduced circulating homocysteine levels of red drum also has been observed in multiple trials. Supplemental creatine also was demonstrated to have osmoregulatory benefits in hybrid striped bass when cultured in brackish water. The beneficial effects of dietary creatine supplementation on channel catfish fry also was noted when reared in brackish (10 ppt) water. Results from these various studies emphasize the potential importance of a number of synthesizable amino acids and other nitrogenous compounds in the nutrition and metabolism of various fish species.